Grid support for electron tubes



Dec. 1, 1970 F. cs. HAMMERSAND ET AL 3,544,831

can: surronw FOR ELECTRON TUBES Filed Oct 23, 19s? 2 Sheets-Sheet 1 ATTORNEY Dec. 1, 1970 F. G. HAMMERSAND ET AL 3,544,831

GRID SUPPORT FOR ELECTRON TUBES 2 Sheets-Sheet 2 7 Filed Oct. 23, 1967 K'rwwv a w 2 M sfl K #0 r A. r 4 4 1 4 4 ITTOIUEY United States Patent US. Cl. 313-348 12 Claims ABSTRACT OF THE DISCLOSURE A grid support structure for an electron tube has an elongated finlike grid support adapted to support two ad acent edge regions of two adjacent grid sections, and a two thin strips of a material having a relatively low coeflicient of thermal expansion brazed one to the top and the other to the bottom of each grid section. A longitudinal slot extends into an end region of the finlake supports to permit independent bending movement of portions of the two sides of the finlike support and thereby isolates the thermally induced movements of the two grid sections. The finlike grid support also has longitudinal slots in the sides thereof perpendicular to the slot in the end region to facilitate independent bending movements of the support portions referred to. The thin strips of material on the top and bottom of the grid sections prevent excessive contraction which would otherwise result when the grid sections cool after brazing.

BACKGROUND OF THE INVENTION Field of the invention This invention relates in general to improved means for mounting a grid in a high power electron tube and particularly to an improved grid support for such an electron tube.

Description of the prior art A high power tube of the general type described in Pat. No. 2,817,031, to Wilfred P. Bennett includes a pluraltiy of elementary tetrodes arranged about the central axis of the tube to provide high current densities at elevated frequencies. To form these elementary tetrodes, a cathode, control grid and screen grid of the tube each include a plurality of separate elements spaced in annular arrays about a single centrally located cylindrical anode. Moving outward from the cylindrical anode, the elementary tetrodes consist of an array of screen grids adjacent to the anode, an array of control grids located directly behind the screen grids and an array of cathodes located closely behind the control grids.

The elementary screen grids comprise a plurality of wires attached to screen grid windows which are supported by a screen grid assembly, which includes a cy1indrical screen grid ring, and a plurality of screen grid supports. The cylindrical screen grid ring surrounds and is coaxial with the cylindrical anode. The screen grid supports are finlike structures which are attached at one end to the screen grid ring and extend inwardly from the screen grid ring toward the anode. The combination of the cylindrical screen grid ring and the finlike screen grid supports forms the sides of a plurality of electrode cavities in which the cathodes, screen grids and control grids of the tube are mounted to form the elementary tetrodes. The screen grid windows are rectangular frames having a central rectangular aperture and are mounted across the open end of the electrode cavities adjacent to the anode and between two adjacent screen grid supports. The screen grid wires are attached as by brazing across the central rectangular aperture in the screen grid window.

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The screen grid windows and the attached screen grid wires are slidably mounted in longitudinal slots in the side walls of the finlike screen grid supports at a point close to the inner end of the screen grid supports so that the screen grid is adjacent to the anode and between the anode and the control grid.

In prior art high power tubes of this type, the screen grid windows were attached to the screen grid supports by brazing them into longitudinal slots formed in the side wall of the screen grid supports; but changes in the size of the tubes necessitated by requirements for higher tube performance made brazing impractical. In order to achieve high radio frequency power output levels and wider operating bandwidths in this type tube, it became necessary to increase the size of the elementary tetrodes which in turn required larger electrodes and a larger electrode cavity. The elements of the screen grid assembly had to be correspondingly increased in size. When attempts were made to braze the enlarged screen grid windows into the slots in the screen grid supports, the screen grid wires bowed because the various adjacent parts of the tube such as the screen grid wires, windows and supports expanded at diiferent rates due to the heat resulting from brazing and contracted at diifering rates as they cooled. This problem of differing rates of thermal expansion and contraction was always present when the screen grid windows were brazed to the supports but it became worse in the enlarged tube structure because longer screen grid wires were needed to extend across the enlarged screen grid windows making heat dissipation from the wires more difficult.

One way to avoid this problem is to use the same material for the screen grid wires, windows and supports to insure a constant rate of thermal expansion and contraction throughout the screen grid assembly, but no one material is suitable for all the parts involved. Copper is customarily used in fabricating the screen grid supports to provide good thermal conductivity since the screen grid supports serve as heat sinks for the screen grid wires. Because the screen grid wire temperature during tube operation would cause excessive thermal expansion and bowing in copper wires, copper cannot be used for the screen grid wires. Tungsten is a good material for the screen grid wires because it has a low coefiicient of thermal expansion, but tungsten is not a sufficiently good thermal conductor to be used in the screen grid supports.

In another effort to avoid the problems created by the brazing heat, the screen grid windows were attached to the screen grid supports by peening techniques, but this resulted in the screen grid wires overheating during CW operation. Peening worked satisfactorily in tubes designed for short pulse operation but proved to be unsatisfactory in tubes which were designed for CW operation. -In CW operation, the screen grid wires become excessively hot when the screen grid window is attached to the screen grid support by peening because peening does not provide an adequate thermal contact for heat conduction from the screen grid wires to the screen grid supports which operate as heat sinks. Because of the lack of an effective heat sink, the screen grid wires overheated during tube operation, causing the wires to bow and often to short out against the control grid wires which are located closely behind the screen grid. The excessive heat can also cause the screen grid wires to melt at their centers, where the heating is greatest, because this point is fartherest from the heat sink. Any overheating causes a permanent change in the shape of the screen grid wires which distorts the electron optics of the tube.

A further source of screen grid wire distortion during brazing is the presence of cross members which provide structural strength at the top and bottom of the copper screen grid windows. Since copper has a high coefficient of thermal expansion, these cross members expand a relatively large amount due to the heat which is applied to the screen grid windows in order to braze the windows to the screen grid supports. As the screen grid windows cool, these copper cross members contract causing the screen grid wires close to the top and bottom of the windows to become bowed. This bowing occurs because the tungsten screen grid wires have a lower coefiicient of thermal expansion than the adjacent copper cross members.

SUMMARY OF THE INVENTION The above described difficulties in the prior art are overcome by an improved screen grid support which provides enough flexibility to compensate for the differing rates of thermal expansion of the screen grid wires, windows and supports, and thereby prevents the distortion of the screen grid wires which would otherwise result from the heat used in brazing the screen grid windows to the screen grid supports. The improved screen grid supports are finlike structures extending radially inwardly from the cylindrical screen grid ring toward the central cylindrical anode of the tube. These screen grid supports are attached to the screen grid ring at one edge and have a free edge adjacent to the anode. A longitudinal edge slot is formed in the free edge of the finlike supports parallel to the tube axis. A pair of opposed longitudinal window slots are formed in the opposed sides of the finlike support perpendicular to the edge slots. The screen grid windows are slidably mounted in these window slots and are supported by two adjacent screen grid supports. In addition to the window slots a plurality of longitudinal side slots are formed in the opposed sides of the finlike supports. These side slots are perpendicular to the edge slot and parallel to the window slots. The presence of the edge slot and the side slots in the screen grid support permits the support to bend allowing the screen grid wires to expand axially during brazing and thereby prevents bowing and distortion of the wires.

In addition to the inclusion of the above described slots in the finlike supports, the screen grid support structure is further improved by brazing two thin pieces of a metal which has a low coeflicient of thermal expansion such as molybdenum or tungsten onto each screen grid window, one piece across the outer edge of the top cross member and the other piece across the bottom cross member of the screen grid window. These metal members are brazed onto the screen grid window while the window is still hot from being brazed to the finlike supports. The metal members prevent the copper cross members from contracting excessively as they cool, and causing the screen grid wires near the top and bottom of the screen grid windows to bow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of a portion of the electrode structure of a tube showing the improved screen grid supports;

FIG. 2 is a partially cut away view showing the improved screen grid supports and parts of the electrode structure; and

FIG. 3 is a perspective view of the screen grid assembly of an electron tube showing the improved screen grid supports.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, '2 and 3 show a portion of the electrode structure of a tetrode power tube employing the improved screen grid supports of this invention. A cylindrical anode 12 in FIG. 1 is located at the center of the tube. A massive cylindrical screen grid ring 14 surrounds and is coaxial with the anode 12. Finlike screen grid supports 16 are attached to the screen grid ring 14 at one edge and extend inwardly toward the central anode 12. Screen grid windows 18, which are rectangular frames having supporting cross members 19 at the top and bottom as shown in FIG. 3 and a central rectangular aperture 20, are mounted between two adjacent screen grid supports 16 at a point which is adjacent to the anode 12. Two thin pieces 21 of metal which has a low coefficient of thermal expansion such as molybdenum or tungsten are brazed along the outer edges of the supporting cross members 19 of the screen grid windows 18. Screen grid wires 22 are fixed as by brazing or peening across the rectangular aperture in the screen grid windows 18. The combination of the screen grid ring 14 and the screen grid supports 16 forms a plurality of electrode cavities 24 within which all the electrodes other than the anode of the tube are mounted. The control grid support 26 is a rectangular structure extending radially outwardly from the screen grid wires 22 and having a cavity 28 adjacent to the control grid wires 30. Control grid wires 30 are fixed as by brazing across the sides defining the cavity 28. The control grid support 26 is located between the screen grid supports 16 within the electrode cavity 24 so that the control grid wires 30 are behind the screen grid wires 22 as shown in FIG. 2. A cathode 32 is located behind the control grid wires 30 within the cavity 28 in the control grid support 26.

A plurality of slots which are seen best in FIG. 2 are formed in the screen grid supports 16. A first pair of longitudinal opposed window slots 34 and 36, as shown in FIG. 2 are formed in sides 37, 39 of the finlike screen grid supports 16 parallel to the tube axis and adjacent to a free edge 40 of the screen grid supports 16. The screen grid windows 18 are brazed into these window slots 34 and 36 and are thereby supported between two adjacent screen grid supports 16. In the embodiment depicted in FIG. 3, a single edge slot 41 extends into the free edge 40 of the screen grid supports 16. This edge slot 41 is formed to extend parallel and at right angles to window slots 34 and 36 and efifectively divides the free edge area of the screen grid supports 16 into two independent vanes 42 and 43 (FIG. 3). A second pair of opposed longitudinal slots 44 and 46, as shown in FIG. 3, extend parallel and at right angles to edge slot 41 and are closer to the fixed edge of the finlike grid support 16 than the window slots 34 and 36. The presence of the edge slot 41 and the side slots 44 and 46 protects the screen grid wires 22 from distortion due to thermal stresses caused by the heat involved in brazing the screen grid windows 18 into the window slots 34 and 36 in the screen grid supports 16.

During fabrication of the screen grid assembly, the screen grid windows 18 are slidably mounted in the window slots 34 and 36 in the screen grid supports 16. The screen grid windows 18 are then brazed into the window slots 34 and 36 to hold them in place across the electrode cavity 24. Brazing the screen grid windows 18 into the screen grid supports 16 provides a good path for thermal conductivity from the screen grid wires 22 and the screen grid windows 18 to the screen grid supports 16 which act as heat sinks during tube operation.

The heat required for brazing the screen grid windows 18 to the screen supports 16 causes the screen grid wires 22, windows 18 and supports 16 to expand. Since the screen grid windows 18 are made of copper, a material having a high coefi'icient of thermal expansion, the screen grid windows expand more than the screen grid wires 22 which are attached to them. This expansion is most pronounced in the cross members 19 since a greater expanse of copper is present in these two areas. As the cross members 19 expand, the screen grid wires 22 are stretched and as the windows 18 cool they contract, causing the wires 22 to bow. To prevent this, the two thin pieces 21 of a metal having a low coelficient of thermal expansion, such as molybdenum or tungsten, are brazed to the cross members 19 while they are hot and before the copper has cooled and contracted. The presence of these metal pieces 21 prevents the copper cross members 19 from fully contracting and thereby prevents the adjacent screen grid wires 22 from bowing.

During the brazing cycle and after the braze has solidified, the screen grid wires 22 exert an axial force outward toward the screen grid supports 16 on each side. The right-hand independent vane 43 (FIG. 3) bends slightly about a point between a side of the edge slot 41 and the bottom of the side slot 44 in response to the thermal expansion of one of the screen grid windows 18, while the left-hand independent vane 42 bends slightly about a point between a side of the edge slot 41 and the bottom of the side slot 46 in response to axial pressure from the adjacent screen grid window. In this way, independent vanes 42 and 43 of each screen grid support 16 is rotated or bent an amount which is approximately equal to onehalf of the magnitude of the expansion of each screen grid structure comprising a combination of screen grid window and wires.

We claim:

1. An electrode support for an electron tube comprising:

(a) a plurality of finlike support structures each having two opposed sides and a free edge;

(b) said free edge having a longitudinal edge slot extending therealong; and

(c) said opposed sides having a plurality of opposed longitudinal slots therein in perpendicular relation to said edge slot.

2. An electrode support as described in claim 1 wherein said plurality of finlike support structures are attached to an annular support.

3. An electrode support as described in claim 1 wherein said plurality of longitudinal slots are adjacent to said edge slot.

4. An electrode support as described in claim 3 wherein said plurality of longitudinal slots comprises a plurality of pairs of mutually opposed slots.

5. In an electron tube having a cylindrical anode circularly arranged about an axis and having coaxial concentric annular arrays of screen, control and cathode electrodes mounted in that sequence about said anode, a screen electrode support structure comprising:

(a) a cylindrical conductive ringlike support member coaxial with and surrounding said electrodes;

(b) a plurality of rectangular screen grid windows having two elongated parallel sides and two shorter parallel cross members;

() a plurality of conductive finlike support members attached to said ringlike support member and ex tending radially inwardly toward said anode and having a free edge extending parallel to said axis;

(d) each of said finlike support members having a radially extending longitudinal edge slot in said free edge parallel to said axis to provide flexibility and prevent damage to the screen electrode resulting from thermal stresses;

(e) each of said finlike support members having a windows slot in opposite surfaces thereon perpendicular to said radially extending edge slot for slidably receiving said elongated parallel sides of said screen grid windows; and

(f) each of said finlike support members having a plurality of side slots perpendicular to said radially extending slot and parallel to said window slots to provide flexibility and prevent damage to the screen electrode resulting from thermal stresses.

6. A screen electrode support structure as described in claim 5 wherein said plurality of side slots are arranged in pairs with their bottoms in mutual opposition.

7. A screen electrode support structure as described in claim 5 wherein said plurality of pairs of side slots and said window slots are adjacent to said radially extending edge slot.

8. A screen electrode support structure as described in claim 7 wherein said plurality of side slots comprise a single pair of mutually opposed side slots.

9. A screen electrode support structure as described in claim 5 wherein all of said slots in said electrode support structure have a rectangular cross-section.

10. A screen electrode support structure as described in claim 5 wherein said window slots are closer to said free edge than said side slots.

11. A screen electrode support structure as described in claim 5 wherein said screen grid windows have a thin piece of a metal having a low coefficient of thermal expansion brazed to each of said shorter parallel cross members while said cross members are at an elevated temperature.

12. An electrode support for an electron tube comprising:

(a) a rectangular screen grid window having two elongated parallel sides and two shorter parallel cross members, said sides and said cross members being made of a heat-conductive material having a predetermined coefiicient of thermal expansion;

(b) two metal members each having a lower coefiicient of thermal expansion than said predetermined co efficient of expansion, one of said metal members being brazed to one of said parallel cross members and the other of said metal members being brazed to the other of said parallel cross members; and

(0) means to support said rectangular screen grid window within said electron tube.

References Cited UNITED STATES PATENTS 4/1967 Iwayanagi 3l3-350 7/1958 Hoover 3l3-307 US. Cl. X.R. 

